1. Field of the Invention
The invention relates to a method of polishing disks, such as glass disks used as data storage devices.
2. Background Information
Circular-shaped magnetic disks are typically used in hard disk drives of computers, for example, for use in data storage applications. Such magnetic disks may be formed from aluminum or from glass, for example, and will typically have a magnetic surface coating located thereon. A head of the disk drive interacts with the magnetic surface coating to read and write information to the disk. Such magnetic disks have achieved storage capacities of several gigabytes or more, using current technology.
Typically, the head of the disk drive that reads and writes information to the disk is arranged to float a small distance above a surface of the disk. By bringing the head closer to the surface of the disk, higher density recording becomes possible.
As mentioned, often the magnetic disks are formed from aluminum. However, aluminum is relatively soft, so when it is handled, it is possible to ding the disk and form an area where data cannot be retrieved.
Further, the aluminum is typically coated with a nickel plating to give the disk more stiffness and a harder surface. However, the nickel plating has a tendency to become magnetic, causing errors in reading and writing to the disk.
Additionally, aluminum disks are limited in how smooth their surfaces can be made. The smoother a magnetic disk can be made, the closer the head can be brought to the surface of the disk during the read/write operations.
To overcome the problems associated with aluminum disks, attention has been directed to the utilization of glass disks. For example, the disks may be amorphous glass disks formed from sodium lithium or aluminosilicate glass.
Typically, computer manufacturers purchase blank glass disks, for example from a glass manufacturer. Once received, the computer manufacturer may subject the blank glass disks to various processes to prepare the glass disks for use as data storage devices. For example, the computer manufacturer may polish the blank glass disksxe2x80x94to remove surface scratches from the disks, and to planarize the surfaces of the disks to remove any waviness. Planarizing the surfaces of the disks provides for a smoother surface finish, allowing a head of an associated disk drive to be brought closer to the surface of a respective disk. By bringing the head closer to the surface of the disk, higher density recording becomes possible.
A typical machine used for polishing the disks includes two superposed platens respectively disposed over and under one or more of the disks, so that opposing surfaces of the disks can be polished simultaneously. Moreover, the polishing machine may include carriers that position and retain the disks during the polishing operation. Such carriers may be adapted to rotate relative to the platens. For example, the polishing machine may also include an outer ring gear, disposed around an outer periphery of the platens, and an inner gear, that projects through a hole formed in a center of the platens. The carriers can then have a toothed outer periphery, which engages with the teeth or pins of the outer ring gear and the teeth or pins of the inner gear. Rotation of the inner gear and outer gear in opposite directions, for example, thus causes the carrier to rotate globally around the inner gear, and about an axis of the carrier.
In order to protect the surfaces of the disks from being damaged by the hard surfaces of the platens during the polishing process, and to help retain a polishing slurry in contact with the surfaces of the disks, the polishing machine typically has a so-called polishing pad on a surface of each platen. A respective polishing pad thus separates the surface of the platen from the surface of the disk during the polishing operation.
Typically, the manufacturer of the polishing machine will polish the surfaces of the platens using a lapping technique, prior to the polishing machine being shipped to the end user. It is conventionally believed that the lapping technique provides the platens with a relatively flat and planar surface suitable for most polishing operations.
To polish the disks, the polishing slurry is provided on a surface of the disks. The platens are brought together to exert a predetermined pressure upon the disks, and the carriers and disks are rotated, thus planarizing and polishing the surfaces of the disks.
It has been noted that during the polishing of glass disks, for example, that the polishing pads are subject to increased wear at their inner diameters. Thus, due to this uneven wear, the polishing pads must be replaced at a relatively increased rate. Since the polishing pads are typically adhered to the platens using an adhesive, the replacement of the polishing pads requires that the polishing machine be removed from service for an extended period of time, thus reducing thru-put and increasing the cost of operations.
Thus, there is a need for a way to polish disks, such as glass disks that prevents uneven wear of the polishing pads.
It has further been observed that during the polishing of glass disks, for example, that the teeth of the carriers tend to wear prematurely. In fact, the teeth can become so worn that they will shear off from the carrier, causing the polishing machine to become inoperative (i.e., a so-called mid-cycle crash). As will be appreciated, since the carriers are relatively expensive, a long life is desirable. Moreover, mid-cycle crashes require that the polishing machine be removed from service for an extended period of time, thus reducing thru-put and increasing the cost of operations.
Thus, there is a need for a way to polish disks, such as glass disks that prevents premature wear of the carriers.
It has also been observed that the polished glass disks, for example, have thickness variations from the inner diameter to the outer diameter. However, these thickness variations reduce the strength of the disks in their thinner regions, increasing the likelihood that the disks may break and fail. Moreover, the thicker regions of the disks will define how close a head of an associated disk drive can be brought to the surface of the disk. That is, when the head is over the thinner regions of the disk, the head will be further from the surface of the disk than it was when it was over the thicker regions of the disk. This may lead to errors when reading and writing to the disks and/or reduce the density of recording to the disk.
Thus, there is a need for a way to polish disks, such as glass disks, so that the polished disks have a minimum of thickness variations across their respective surfaces.
It is, therefore, a principle object of this invention to provide a method of polishing disks.
It is another object of the invention to provide a method of polishing disks that solves the above mentioned problems.
These and other objects of the present invention are accomplished by the method of polishing disks disclosed herein.
In an exemplary aspect of the invention, a glass disk polishing operation is performed using a conventional polishing machine. For example, the Peter Wolters AC320 polishing machine has proven to be suitable for the polishing of the glass disks. Of course, other polishing machines may be used without departing from the spirit and scope of the invention.
In a further exemplary aspect of the invention, the platens of the polishing machine may be lap-polished by the end user, while operating the polishing machine at the same or similar operational variables (temperature, pressure and/or speed) as when polishing the glass disks. For example, the platens can be lapped using a lapping disk and a silicon-carbide compound, while exerting a force of about 500 daN (i.e., a force close to the force used in the polishing operation) over a total surface area similar to the total surface area of the glass disks to be polished.
In another exemplary aspect of the invention, the polishing pads of the polishing machine are dressed to compensate for irregularities in the surface profiles of the platens. It is significantly easier to dress the polishing pads to have a desired profile than it is to lap the platens in the manner described above. For example, whereas it may take up to a week to lap the platens to have a desired profile, the polishing pads can be dressed in as little time as 15 to 30 minutes, for example.
In this exemplary aspect of the invention, the polishing pads are dressed while operating the polishing machine at the same or similar pressure, temperature and/or rotational speed as when the disks are polished. By dressing the polishing pads in this manner, the underlying profile of the platens can be effectively ignored, since the dressed polishing pads will compensate for any irregularities in the surface profile of the platens.
In another exemplary aspect of the invention, the polishing pads are dressed using specifically tailored dressing disks that are disposed within the holes or recesses of the carriers of the polishing machine. The dressing disks have a similar configuration to the glass disks, so that they fit within the respective holes or recesses of the carriers in a manner similar to the glass disks. That is, the dressing disks have a surface area similar to the disks. Further, and similar to the glass disks, the dressing disks are thicker than the carriers, so that the upper and lower surfaces of the respective dressing disks contact the exposed surfaces of the polishing pads. This approach allows the polishing machine to be operated at or near the same conditions used for polishing the glass disks.
In another exemplary aspect of the invention, the force applied by the polishing machine is controlled depending on the number of dressing disks used. By way of example, if using fifty dressing disks in an arrangement adapted to accommodate fifty glass disks, then a force of about 500 daN may be applied for the dressing operation. However, if only thirty dressing disks are used in an arrangement adapted to accommodate fifty glass disks, in order to maintain a similar pressure as when using fifty disks, the force may be reduced to about 300 daN. This ensures that the pressure exerted by the dressing disks is not so great as to destroy the polishing pads.
In a further aspect of the invention, the dressing disks have their outer surfaces imbedded with diamond particles. Due to the hardness of the diamond particles, the dressing disks are resistant to wear during the dressing procedure. The diamond particles thus ensure that the dressing disks retain their dressing ability for a relatively long period of time. Moreover, the diamond particles allow the dressing procedure to be performed without requiring any further dressing compounds, which may contaminate the polishing procedures. However, water may be used during the dressing procedure to reduce heat build-up and to rinse away particles of the polishing pads removed during the dressing procedure.
The dressing disks may further include a stainless steel substrate, which could be plated with a further material that serves as a carrier for the diamond particles. For example, the further material could be a nickel plating. This configuration of the dressing disks has been shown to be able to withstand the high forces and pressures applied during the dressing operation.
In order for the polishing pads to compensate for the surface irregularities in the surface profiles of the platens, the polishing pads advantageously will have a thickness greater than an amount of deviation being compensated for. For example, if the surfaces of the platens deviate from a parallel state by 0.005 inches, then it may be desired to have relatively thick polishing pads, such as ones having a thickness of about 0.05 inches.
In the aforementioned processes, the parameters used during the lapping or dressing operations may be within about 10 percent or closer, for example, 5 percent, of the parameters used during the polishing of the glass disks. Moreover, it is currently believed that the applied pressure exerts the greatest influence on the surface profile of the platens, followed by the temperature.